Do a body kit like this.
Aerocivic - Honda Civic modifications for maximum gas mileage - aerocivic.com
Anything else will take much longer to pay off (if ever) unless you get it for a steal. Most of them are made to look "cool", not for function.
Can you really get 600k miles out of that car? Rust will likely take it before then unless you're driving 50k miles a year. Factor in the auto tranny and you'll likely end up spending more by keeping it that long vs buying a new beater every few years.